Silica aerogel is known as a material having small thermal conductivity and having thermal insulation properties. The silica aerogel is useful as a functional material having excellent functionality (thermal insulation properties, etc.), specific optical properties, specific electric properties, etc., and is used in, for example, electronic substrate materials that exploit the ultralow dielectric properties of the silica aerogel, thermal insulating materials that exploit the high thermal insulation properties of the silica aerogel, and light reflective materials that exploit the ultralow refractive index of the silica aerogel.
A supercritical drying method of hydrolyzing an alkoxysilane, polymerizing the resultant, and drying the obtained gelled compound (alcogel) under supercritical conditions of a dispersion medium is known as a method for manufacturing such silica aerogel (see, for example, Patent Literature 1). The supercritical drying method is a method of introducing alcogel and a dispersion medium (solvent used in drying) into a high-pressure vessel, preparing the dispersion medium into a supercritical fluid by the application of a temperature and a pressure equal to or higher than its critical point, and thereby removing the solvent contained in the alcogel. However, since the supercritical drying method requires a high-pressure process, investment in equipment such as a special apparatus capable of resisting supercriticality is necessary, and furthermore, a great deal of labor and time are necessary.
Accordingly, an approach of drying alcogel by using a general-purpose method that does not require the high-pressure process has been proposed. For example, a method of using a monoalkyltrialkoxysilane and a tetraalkoxysilane in combination at a particular ratio as gel raw materials so that the resulting alcogel is improved in strength and dried at normal pressure is known as such a method (see, for example, Patent Literature 2). However, in the case of adopting such drying under normal pressure, the gel tends to shrink due to stress caused by capillary force in the inside of the alcogel.